There are many circumstances in industrial control and processing in which it is necessary to control linear movement or rotary movement at selectable points on the range between two end points. The traditional approach to this very important operation was typically divided into two functions. The first function involved the physical actuator which would apply force to the element to be moved and to thereby accomplish the actual physical movement from one point on the range of movement to another point. The second function was a controller function typically involving a feed-back loop. Typically, the controller would monitor a signal indicating the position of the element to be moved and that signal would be fed back to a remote control device. The control device would typically be remotely located because it is frequently the case that the actuator and element to be actually moved are in a reasonably hostile environment, frequently not appropriate for the kind of delicate controllers often employed in sophisticated process control. The design of the controller frequently involved relatively complex analysis because of the sometimes complex relationship between the signal indicative of the position of the element to be moved and the actions of the actuator in accomplishing that movement. This complexity in controller design frequently adds very greatly to the cost of developing an appropriate actuator-controller pair for a specific application.
Furthermore, the traditional approach to these movement control problems frequently involve process signal time delays which can result in unstable process control functions and various types of destructive oscillation. This is particularly true of the pneumatic control methods which are preferred in the almost universal circumstances where explosive atmosphere or high environmental moisture argue against electric control.
Finally, there are numerous process control situations where the actuator and any associated equipment must be located in difficult to reach environments frequently involving relatively hostile conditions. Furthermore, many process operations are running constantly, and, therefore, any down-time attributable to maintenance of control actuators or associated equipment is frequently unacceptable. Furthermore, the design criteria for many process situations require reliability of equipment for very long periods of time, imposing further difficulties on what might otherwise be relatively delicate and maintenance-prone devices.
These and other difficulties experienced with prior art devices have been obviated in a novel manner by the present invention. It is, therefore, an outstanding object of the present invention to provide an actuator-controller pair which can respond to a process signal in a feed-forward control loop to carry out specific physical movement along a range between two end points.
Another object of this invention is the provision of an actuator-controller pair which can be constructed from commodity-type equipment in a modular fashion so that the development cost for equipment can be controlled and minimized.
A further object of the present invention is an actuator-controller pair in which the functions are essentially formed into one unit to substantially eliminate control signal time delays between the actuator and the controller thereby reducing various destructive effects of control time delays.
A still further object of the invention is the provision of a controller-actuator pair which, because it is essentially a single unit, can be quickly and easily installed in its appropriate locations with a minimum of hook-up activity and therefore, a minimum activities of which create a very high probability of potential installation-related problems.
It is a further object of the invention to provide an actuator-controller pair which employs a relatively available and fully developed modular controller unit which directly monitors the physical positioning of the actuator to control the actuator.
A still further object of the present invention is the provision of an actuator-controller pair in which problems concerning destructive oscillation and resulting early failure of the equipment have been controlled or eliminated.
With these and other objects in view, as will be apparent to those skilled in the art, the invention resides in the combination of parts set forth in the specification and covered by the claims appended hereto.